Global motion constrained motion vector in inter prediction

ABSTRACT

A decoder includes circuitry configured to receive a bitstream, extract a header associated with a current frame and including a signal characterizing that global motion is enabled and further characterizing parameters of a global motion model, and decoding the current frame, the decoding including using a motion model for each current block having a complexity that is less than or equal to a complexity of the global motion model. Related apparatus, systems, techniques, and articles are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of International Application No. PCT/US 20/29926, filed on Apr. 24, 2020 and entitled “GLOBAL MOTION CONSTRAINED MOTION VECTOR IN INTER PREDICTION,” which claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 63/838,563, filed on Apr. 25, 2019, and titled “GLOBAL MOTION CONSTRAINED MOTION VECTOR IN INTER PREDICTION.” Each of International Application No. PCT/US 20/29926 and U.S. Provisional Patent Application Ser. No. 63/838,563 is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of video compression. In particular, the present invention is directed to global motion constrained motion vector in inter prediction.

BACKGROUND

A video codec can include an electronic circuit or software that compresses or decompresses digital video. It can convert uncompressed video to a compressed format or vice versa. In the context of video compression, a device that compresses video (and/or performs some function thereof) can typically be called an encoder, and a device that decompresses video (and/or performs some function thereof) can be called a decoder.

A format of the compressed data can conform to a standard video compression specification. The compression can be lossy in that the compressed video lacks some information present in the original video. A consequence of this can include that decompressed video can have lower quality than the original uncompressed video because there is insufficient information to accurately reconstruct the original video.

There can be complex relationships between the video quality, the amount of data used to represent the video (e.g., determined by the bit rate), the complexity of the encoding and decoding algorithms, sensitivity to data losses and errors, ease of editing, random access, end-to-end delay (e.g., latency), and the like.

Motion compensation can include an approach to predict a video frame or a portion thereof given a reference frame, such as previous and/or future frames, by accounting for motion of the camera and/or objects in the video. It can be employed in the encoding and decoding of video data for video compression, for example in the encoding and decoding using the Motion Picture Experts Group (MPEG)-2 (also referred to as advanced video coding (AVC) and H.264) standard. Motion compensation can describe a picture in terms of the transformation of a reference picture to the current picture. The reference picture can be previous in time when compared to the current picture, from the future when compared to the current picture. When images can be accurately synthesized from previously transmitted and/or stored images, compression efficiency can be improved.

SUMMARY OF THE DISCLOSURE

In an aspect, a decoder includes circuitry configured to receive a bitstream, extract a header associated with a current frame and including a signal characterizing that global motion is enabled and further characterizing parameters of a global motion model, and decode the current frame, the decoding including using a motion model for each current block having a complexity that is less than or equal to a complexity of the global motion model.

In another aspect, a method includes receiving, by a decoder, a bitstream. The method includes extracting a header associated with a current frame and including a signal characterizing that global motion is enabled and further characterizing parameters of a motion model. The method includes decoding the current frame, the decoding including using a motion model for each current block having a complexity that is less than or equal to a complexity of the global motion model.

These and other aspects and features of non-limiting embodiments of the present invention will become apparent to those skilled in the art upon review of the following description of specific non-limiting embodiments of the invention in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein:

FIG. 1 is a diagram illustrating motion vectors of an example frame with global and local motion;

FIG. 2 illustrates three example motion models that can be utilized for global motion including their index value (0, 1, or 2);

FIG. 3 is a process flow diagram according to some example implementations of the current subject matter;

FIG. 4 is a system block diagram of an example decoder according to some example implementations of the current subject matter;

FIG. 5 is a process flow diagram according to some example implementations of the current subject matter;

FIG. 6 is a system block diagram of an example encoder according to some example implementations of the current subject matter; and

FIG. 7 is a block diagram of a computing system that can be used to implement any one or more of the methodologies disclosed herein and any one or more portions thereof.

The drawings are not necessarily to scale and may be illustrated by phantom lines, diagrammatic representations and fragmentary views. In certain instances, details that are not necessary for an understanding of the embodiments or that render other details difficult to perceive may have been omitted.

DETAILED DESCRIPTION

“Global motion” in video refers to motion and/or a motion model common to all pixels of a region, where a region may be a picture, a frame, or any portion of a picture or frame such as a block, CTU, or other subset of contiguous pixels. Global motion may be caused by camera motion; for example, camera panning and zooming may create motion in a frame that may typically affect the entire frame. Motion present in portions of a video may be referred to as local motion. Local motion may be caused by moving objects in a scene, such as without limitation an object moving from left to right in the scene. Videos may contain a combination of local and global motion. Some implementations of the current subject matter may provide for efficient approaches to communicate global motion to a decoder and use of global motion vectors in improving compression efficiency.

FIG. 1 is a diagram illustrating motion vectors of an example frame 100 with global and local motion. Frame 100 includes a number of blocks of pixels illustrated for exemplary purposes as squares, and their associated motion vectors illustrated as arrows. Squares (e.g., blocks of pixels) with arrows pointing up and to the left indicate blocks with motion that may be considered to be global motion and squares with arrows pointing in other directions (indicated by 104) may indicate blocks with local motion. In the illustrated example of FIG. 1, many blocks have same global motion. Signaling global motion in a header, such as a picture parameter set (PPS) or sequence parameter set (SPS) and using signaled global motion may reduce motion vector information needed by blocks and may result in improved prediction. Although for illustrative purposes examples described below refer to determination and/or application of global or local motion vectors at a block level, global motion vectors may be determined and/or applied for any region of a frame and/or picture, including regions made up of multiple blocks, regions bounded by any geometric form such as without limitation regions defined by geometric and/or exponential coding in which one or more lines and/or curves bounding the shape may be angled and/or curved, and/or an entirety of a frame and/or picture. Although signaling is described herein as being performed at a frame level and/or in a header and/or parameter set of a frame, signaling may alternatively or additionally be performed at a sub-picture level, where a sub-picture may include any region of a frame and/or picture as described above.

As an example, and still referring to FIG. 1, simple translational motion may be described using a motion vector (MV) with two components MVx, MVy that describes displacement of blocks and/or pixels in a current frame. More complex motion such as rotation, zooming, and warping may be described using affine motion vectors, where an “affine motion vector,” as used in this disclosure, is a vector describing a uniform displacement of a set of pixels or points represented in a video picture and/or picture, such as a set of pixels illustrating an object moving across a view in a video without changing apparent shape during motion. Some approaches to video encoding and/or decoding may use 4-parameter or 6-parameter affine models for motion compensation in inter picture coding.

For example, a six-parameter affine motion can be described as: x′=ax+by+c y′=dx+ey+f

A four-parameter affine motion can be described as: x′=ax+by+c y′=−bx+ay+f where (x,y) and (x′,y′) are pixel locations in current and reference pictures, respectively; a, b, c, d, e, and f are parameters of an affine motion model.

Still referring to FIG. 1, parameters used to describe affine motion may be signaled to a decoder to apply affine motion compensation at the decoder. In some approaches, motion parameters may be signaled explicitly or by signaling translational control point motion vectors (CPMVs) and then deriving affine motion parameters from the translational control point motion vectors. Two control point motion vectors (CPMVs) may be utilized to derive affine motion parameters for a four-parameter affine motion model and three control point translational motion vectors (CPMVs) may be utilized to obtain parameters for a six-parameter motion model. Signaling affine motion parameters using control point motion vectors may allow use of efficient motion vector coding methods to signal affine motion parameters.

In some implementations, and continuing to refer to FIG. 1, global motion signaling may be included in a header, such as a PPS or SPS. Global motion may vary from picture to picture. Motion vectors signaled in picture headers may describe motion relative to previously decoded frames. In some implementations, global motion may be translational or affine. Motion model (e.g., number of parameters, whether the model is affine, translational, or other) used may also be signaled in a picture header. FIG. 2 illustrates three example motion models 200 that may be utilized for global motion including their index value (0, 1, or 2).

Still referring to FIG. 2, PPSs may be used to signal parameters that can change between pictures of a sequence. Parameters that remain the same for a sequence of pictures may be signaled in a sequence parameter set to reduce a size of PPS and reduce video bitrate. An example picture parameter set (PPS) is shown in table 1:

Descriptor pic_parameter_set_rbsp( ) { pps_pic_parameter_set_id ue(v) pps_seq_parameter_set_id u(4) mixed_nalu_types_in_pic_flag u(1) pic_width_in_luma_samples ue(v) pic_height_in_luma_samples ue(v) pps_conformance_window_flag u(1) if( pps_conformance_window_flag ) { pps_conf_win_left_offset ue(v) pps_conf_win_right_offset ue(v) pps_conf_win_top_offset ue(v) pps_conf_win_bottom_offset ue(v) } scaling_window_explicit_signalling_flag u(1) if( scaling_window_explicit_signalling_flag ) { scaling_win_left_offset ue(v) scaling_win_right_offset ue(v) scaling_win_top_offset ue(v) scaling_win_bottom_offset ue(v) } output_flag_present_flag u(1) subpic_id_mapping_in_pps_flag u(1) if( subpic_id_mapping_in_pps_flag ) { pps_num_subpics_minus1 ue(v) pps_subpic_id_len_minus1 ue(v) for( i = 0; i <= pps_num_subpic_minus1; i++ ) pps_subpic_id[ i ] u(v) } no_pic_partition_flag u(1) if( !no_pic_partition_flag ) { pps_log2_ctu_size_minus5 u(2) num_exp_tile_columns_minus1 ue(v) num_exp_tile_rows_minus1 ue(v) for( i = 0; i <= num_exp_tile_columns_minus1; i++ ) tile_column_width_minus1[ i ] ue(v) for( i = 0; i <= num_exp_tile_rows_minus1; i++ ) tile_row_height_minus1[ i ] ue(v) if( NumTilesInPic > 1 ) rect_slice_flag u(1) if( rect_slice_flag ) single_slice_per_subpic_flag u(1) if( rect_slice_flag && !single_slice_per_subpic_flag ) { num_slices_in_pic_minus1 ue(v) if( num_slices_in_pic_minus1 > 0 ) tile_idx_delta_present_flag u(1) for( i = 0; i < num_slices_in_pic_minus1; i++ ) { if( NumTileColumns > 1 ) slice_width_in_tiles_minus1[ i ] ue(v) if( NumTileRows > 1 && ( tile_idx_delta_present_flag | | SliceTopLeftTileIdx[ i ] % NumTileColumns = = 0 ) ) slice_height_in_tiles_minus1[ i ] ue(v) if( slice_width_in_tiles_minus1[ i ] = = 0 && slice_height_in_tiles_minus1[ i ] = = 0 && RowHeight[ SliceTopLeftTileIdx[ i ] / NumTileColumns ] > 1 ) { num_exp_slices_in_tile[ i ] ue(v) for( j = 0; j < num_exp_slices_in_tile[ i ]; j++ ) exp_slice_height_in_ctus_minus1[ i ][ j ] ue(v) i += NumSlicesInTile[ i ] − 1 } if( tile_idx_delta_present_flag && i < num_slices_in_pic_minus1 ) tile_idx_delta[ i ] se(v) } } loop_filter_across_tiles_enabled_flag u(1) loop_filter_across_slices_enabled_flag u(1) } cabac_init_present_flag u(1) for( i = 0; i < 2; i++ ) num_ref_idx_default_active_minus1[ i ] ue(v) rpl1_idx_present_flag u(1) init_qp_minus26 se(v) cu_qp_delta_enabled_flag u(1) pps_chroma_tool_offsets_present_flag u(1) if( pps_chroma_tool_offsets_present_flag ) { pps_cb_qp_offset se(v) pps_cr_qp_offset se(v) pps_joint_cbcr_qp_offset_present_flag u(1) if( pps_joint_cbcr_qp_offset_present_flag ) pps_joint_cbcr_qp_offset_value se(v) pps_slice_chroma_qp_offsets_present_flag u(1) pps_cu_chroma_qp_offset_list_enabled_flag u(1) } if( pps_cu_chroma_qp_offset_list_enabled_flag ) { chroma_qp_offset_list_len_minus1 ue(v) for( i = 0; i <= chroma_qp_offset_list_len_minus1; i++ ) { cb_qp_offset_list[ i ] se(v) cr_qp_offset_list[ i ] se(v) if( pps_joint_cbcr_qp_offset_present_flag ) joint_cbcr_qp_offset_list[ i ] se(v) } } pps_weighted_pred_flag u(1) pps_weighted_bipred_flag u(1) deblocking_filter_control_present_flag u(1) if( deblocking_filter_control_present_flag ) { deblocking_filter_override_enabled_flag u(1) pps_deblocking_filter_disabled_flag u(1) if( !pps_deblocking_filter_disabled_flag ) { pps_beta_offset_div2 se(v) pps_tc_offset_div2 se(v) pps_cb_beta_offset_div2 se(v) pps_cb_tc_offset_div2 se(v) pps_cr_beta_offset_div2 se(v) pps_cr_tc_offset_div2 se(v) } } rpl_info_in_ph_flag u(1) if( deblocking_filter_override_enabled_flag ) dbf_info_in_ph_flag u(1) sao_info_in_ph_flag u(1) alf_info_in_ph_flag u(1) if( ( pps_weighted_pred_flag | | pps_weighted_bipred_flag ) && rpl_info_in_ph_flag ) wp_info_in_ph_flag u(1) qp_delta_info_in_ph_flag u(1) pps_ref_wraparound_enabled_flag u(1) if( pps_ref_wraparound_enabled_flag ) pps_ref_wraparound_offset ue(v) picture_header_extension_present_flag u(1) slice_header_extension_present_flag u(1) pps_extension_flag u(1) if( pps_extension_flag) while( more_rbsp_data( ) ) pps_extension_data_flag u(1) rbsp_trailing_bits( ) }

Additional fields may be added to the PPS to signal global motion. In case of global motion, presence of global motion parameters in a sequence of pictures may be signaled in a SPS and a PPS may reference the SPS by SPS ID. SPS in some approaches to decoding may be modified to add a field to signal presence of global motion parameters in SPS. For example, a one-bit field may be added to the SPS. If global_motion_present bit is 1, global motion related parameters may be expected in a PPS. If global_motion_present bit is 0, no global motion parameter related fields may be present in PPS. For example, the PPS of table 1 may be extended to include a global_motion_present field, for example, as shown in table 2:

Descriptor sequence_parameter_set_rbsp( ) { sps_sequence_parameter_set_id ue(v) . . global_motion_present u(1) rbsp_trailing_bits( ) }

Similarly, the PPS can include a pps_global_motion_parameters field for a frame, for example as shown in table 3:

Descriptor pic_parameter_set_rbsp( ) { pps_pic_parameter_set_id ue(v) pps_seq_parameter_set_id ue(v) . . . pps_global_motion_parameters ( ) rbsp_trailing_bits( ) }

In more detail, a PPS may include fields to characterize global motion parameters using control point motion vectors, for example as shown in table 4:

Descriptor pps_global_motion_parameters ( ) { motion_model_used u(2) mv0_x se(v) mv1_y se(v) if(motion_model_used == 1){ mv1_x se(v) mv1_y se(v) } if(motion_model_used == 2){ mv2_x se(v) mv2_y se(v) } }

As a further non-limiting example, Table 5 below may represent an exemplary SPS:

Descriptor seq_parameter_set_rbsp( ) { sps_seq_parameter_set_id u(4) sps_video_parameter_set_id u(4) sps_max_sublayers_minus1 u(3) sps_reserved_zero_4bits u(4) sps_ptl_dpb_hrd_params_present_flag u(1) if( sps_ptl_dpb_hrd_params_present_flag ) profile_tier_level( 1, sps_max_sublayers_minus1 ) gdr_enabled_flag u(1) chroma_format_idc u(2) if( chroma_format_idc = = 3 ) separate_colour_plane_flag u(1) res_change_in_clvs_allowed_flag u(1) pic_width_max_in_luma_samples ue(v) pic_height_max_in_luma_samples ue(v) sps_conformance_window_flag u(1) if( sps_conformance_window_flag ) { sps_conf_win_left_offset ue(v) sps_conf_win_right_offset ue(v) sps_conf_win_top_offset ue(v) sps_conf_win_bottom_offset ue(v) } sps_log2_ctu_size_minus5 u(2) subpic_info_present_flag u(1) if( subpic_info_present_flag ) { sps_num_subpics_minus1 ue(v) sps_independent_subpics_flag u(1) for( i = 0; sps_num_subpics_minus1 > 0 && i <= sps_num_subpics_minus1; i++) { if( i > 0 && pic_width_max_in_luma_samples > CtbSizeY ) subpic_ctu_top_left_x[ i ] u(v) if( i > 0 && pic_height_max_in_luma_samples > CtbSizeY ) { subpic_ctu_top_left_y[ i ] u(v) if( i < sps_num_subpics_minus1 && pic_width_max_in_luma_samples > CtbSizeY ) subpic_width_minus1[ i ] u(v) if( i < sps_num_subpics_minus1 && pic_height_max_in_luma_samples > CtbSizeY ) subpic_height_minus1[ i ] u(v) if( !sps_independent_subpics_flag ) { subpic_treated_as_pic_flag[ i ] u(1) loop_filter_across_subpic_enabled_flag[ i ] u(1) } } sps_subpic_id_len_minus1 ue(v) subpic_id_mapping_explicitly_signalled_flag u(1) if( subpic_id_mapping_explicitly_signalled_flag ) { subpic_id_mapping_in_sps_flag u(1) if( subpic_id_mapping_in_sps_flag ) for( i = 0; i <= sps_num_subpics_minus1; i++ ) sps_subpic_id[ i ] u(v) } } bit_depth_minus8 ue(v) sps_entropy_coding_sync_enabled_flag u(1) if( sps_entropy_coding_sync_enabled_flag ) sps_wpp_entry_point_offsets_present_flag u(1) sps_weighted_pred_flag u(1) sps_weighted_bipred_flag u(1) log2_max_pic_order_cnt_lsb_minus4 u(4) sps_poc_msb_flag u(1) if( sps_poc_msb_flag ) poc_msb_len_minus1 ue(v) num_extra_ph_bits_bytes u(2) extra_ph_bits_struct( num_extra_ph_bits_bytes ) num_extra_sh_bits_bytes u(2) extra_sh_bits_struct( num_extra_sh_bits_bytes ) if( sps_max_sublayers_minus1 > 0 ) sps_sublayer_dpb_params_flag u(1) if( sps_ptl_dpb_hrd_params_present_flag ) dpb_parameters( sps_max_sublayers_minus1, sps_sublayer_dpb_params_flag ) long_term_ref_pics_flag u(1) inter_layer_ref_pics_present_flag u(1) sps_idr_rpl_present_flag u(1) rpl1_same_as_rpl0_flag u(1) for( i = 0; i < rpl1_same_as_rpl0_flag ? 1 : 2; i++ ) { num_ref_pic_lists_in_sps[ i ] ue(v) for( j = 0; j < num_ref_pic_lists_in_sps[ i ]; j++ ) ref_pic_list_struct( i, j ) } if( ChromaArrayType != 0 ) qtbtt_dual_tree_intra_flag u(1) log2_min_luma_coding_block_size_minus2 ue(v) partition_constraints_override_enabled_flag u(1) sps_log2_diff_min_qt_min_cb_intra_slice_luma ue(v) sps_max_mtt_hierarchy_depth_intra_slice_luma ue(v) if( sps_max_mtt_hierarchy_depth_intra_slice_luma != 0 ) { sps_log2_diff_max_bt_min_qt_intra_slice_luma ue(v) sps_log2_diff_max_tt_min_qt_intra_slice_luma ue(v) } sps_log2_diff_min_qt_min_cb_inter_slice ue(v) sps_max_mtt_hierarchy_depth_inter_slice ue(v) if( sps_max_mtt_hierarchy_depth_inter_slice != 0 ) { sps_log2_diff_max_bt_min_qt_inter_slice ue(v) sps_log2_diff_max_tt_min_qt_inter_slice ue(v) } if( qtbtt_dual_tree_intra_flag ) { sps_log2_diff_min_qt_min_cb_intra_slice_chroma ue(v) sps_max_mtt_hierarchy_depth_intra_slice_chroma ue(v) if( sps_max_mtt_hierarchy_depth_intra_slice_chroma != 0 ) { sps_log2_diff_max_bt_min_qt_intra_slice_chroma ue(v) sps_log2_diff_max_tt_min_qt_intra_slice_chroma ue(v) } } sps_max_luma_transform_size_64_flag u(1) if( ChromaArrayType != 0 ) { sps_joint_cbcr_enabled_flag u(1) same_qp_table_for_chroma u(1) numQpTables = same_qp_table_for_chroma ? 1 : ( sps_joint_cbcr_enabled_flag ? 3 : 2 ) for( i = 0; i < numQpTables; i++ ) { qp_table_start_minus26[ i ] se(v) num_points_in_qp_table_minus1[ i ] ue(v) for( j = 0; j <= num_points_in_qp_table_minus1[ i ]; j++ ) { delta_qp_in_val_minus1[ i ][ j ] ue(v) delta_qp_diff_val[ i ][ j ] ue(v) } } } sps_sao_enabled_flag u(1) sps_alf_enabled_flag u(1) if( sps_alf_enabled_flag && ChromaArrayType != 0 ) sps_ccalf_enabled_flag u(1) sps_transform_skip_enabled_flag u(1) if( sps_transform_skip_enabled_flag ) { log2_transform_skip_max_size_minus2 ue(v) sps_bdpcm_enabled_flag u(1) } sps_ref_wraparound_enabled_flag u(1) sps_temporal_mvp_enabled_flag u(1) if( sps_temporal_mvp_enabled_flag ) sps_sbtmvp_enabled_flag u(1) sps_amvr_enabled_flag u(1) sps_bdof_enabled_flag u(1) if( sps_bdof_enabled_flag ) sps_bdof_pic_present_flag u(1) sps_smvd_enabled_flag u(1) sps_dmvr_enabled_flag u(1) if( sps_dmvr_enabled_flag ) sps_dmvr_pic_present_flag u(1) sps_mmvd_enabled_flag u(1) sps_isp_enabled_flag u(1) sps_mrl_enabled_flag u(1) sps_mip_enabled_flag u(1) if( ChromaArrayType != 0 ) sps_cclm_enabled_flag u(1) if( chroma_format_idc = = 1 ) { sps_chroma_horizontal_collocated_flag u(1) sps_chroma_vertical_collocated_flag u(1) } sps_mts_enabled_flag u(1) if( sps_mts_enabled_flag ) { sps_explicit_mts_intra_enabled_flag u(1) sps_explicit_mts_inter_enabled_flag u(1) } six_minus_max_num_merge_cand ue(v) sps_sbt_enabled_flag u(1) sps_affine_enabled_flag u(1) if( sps_affine_enabled_flag ) { five_minus_max_num_subblock_merge_cand ue(v) sps_affine_type_flag u(1) if( sps_amvr_enabled_flag ) sps_affine_amvr_enabled_flag u(1) sps_affine_prof_enabled_flag u(1) if( sps_affine_prof_enabled_flag ) sps_prof_pic_present_flag u(1) } sps_palette_enabled_flag u(1) if( ChromaArrayType = = 3 && !sps_max_luma_transform_size_64_flag ) sps_act_enabled_flag u(1) if( sps_transform_skip_enabled_flag | | sps_palette_enabled_flag ) min_qp_prime_ts_minus4 ue(v) sps_bcw_enabled_flag u(1) sps_ibc_enabled_flag u(1) if( sps_ibc_enabled_flag ) six_minus_max_num_ibc_merge_cand ue(v) sps_ciip_enabled_flag u(1) if( sps_mmvd_enabled_flag) sps_fpel_mmvd_enabled_flag u(1) if( MaxNumMergeCand >= 2 ) { sps_gpm_enabled_flag u(1) if( sps_gpm_enabled_flag && MaxNumMergeCand >= 3 ) max_num_merge_cand_minus_max_num_gpm_cand ue(v) } sps_lmcs_enabled_flag u(1) sps_lfnst_enabled_flag u(1) sps_ladf_enabled_flag u(1) if( sps_ladf_enabled_flag ) { sps_num_ladf_intervals_minus2 u(2) sps_ladf_lowest_interval_qp_offset se(v) for( i = 0; i < sps_num_ladf_intervals_minus2 + 1; i++ ) { sps_ladf_qp_offset[ i ] se(v) sps_ladf_delta_threshold_minus1[ i ] ue(v) } } log2_parallel_merge_level_minus2 ue(v) sps_explicit_scaling_list_enabled_flag u(1) sps_dep_quant_enabled_flag u(1) if( !sps_dep_quant_enabled_flag ) sps_sign_data_hiding_enabled_flag u(1) sps_virtual_boundaries_enabled_flag u(1) if( sps_virtual_boundaries_enabled_flag ) { sps_virtual_boundaries_present_flag u(1) if( sps_virtual_boundaries_present_flag ) { sps_num_ver_virtual_boundaries u(2) for( i = 0; i < sps_num_ver_virtual_boundaries; i++ ) sps_virtual_boundaries_pos_x[ i ] u(13) sps_num_hor_virtual_boundaries u(2) for( i = 0; i < sps_num_hor_virtual_boundaries; i++ ) sps_virtual_boundaries_pos_y[ i ] u(13) } } if( sps_ptl_dpb_hrd_params_present_flag ) { sps_general_hrd_params_present_flag u(1) if( sps_general_hrd_params_present_flag ) { general_hrd_parameters( ) if( sps_max_sublayers_minus1 > 0 ) sps_sublayer_cpb_params_present_flag u(1) firstSubLayer = sps_sublayer_cpb_params_present_flag ? 0 : sps_max_sublayers_minus1 ols_hrd_parameters( firstSubLayer, sps_max_sublayers_minus1 ) } } field_seq_flag u(1) vui_parameters_present_flag u(1) if( vui_parameters_present_flag ) vui_parameters( ) /* Specified in ITU-T H.SEI|ISO/IEC 23002-7 */ sps_extension_flag u(1) if( sps_extension_flag ) while( more_rbsp_data( ) ) sps_extension_data_flag u(1) rbsp_trailing_bits( ) }

An SPS table as above may be expanded as described above to incorporate a global motion present indicator as shown in Table 6:

Descriptor sequence_parameter_set_rbsp( ) { sps_sequence_parameter_set_id ue(v) . . . global_motion_present u(1) rbsp_trailing_bits( ) }

Additional fields may be incorporated in an SPS to reflect further indicators as described in this disclosure.

In an embodiment, and still referring to FIG. 2, an sps_affine_enabled_flag in a PPS and/or SPS may specify whether affine model based motion compensation may be used for inter prediction. If sps_affine_enabled_flag is equal to 0, the syntax may be constrained such that no affine model based motion compensation is used in the code later video sequence (CLVS), and inter_affine_flag and cu_affine_type_flag may not be present in coding unit syntax of the CLVS. Otherwise (sps_affine_enabled_flag is equal to 1), affine model based motion compensation can be used in the CLVS.

Continuing to refer to FIG. 2, sps_affine_type_flag in a PPS and/or SPS may specify whether 6-parameter affine model based motion compensation may be used for inter prediction. If sps_affine_type_flag is equal to 0, syntax may be constrained such that no 6-parameter affine model based motion compensation is used in the CLVS, and cu_affine_type_flag may not present in coding unit syntax in the CLVS. Otherwise (sps_affine_type_flag_equal to 1), 6-parameter affine model based motion compensation may be used in CLVS. When not present, the value of sps_affine_type_flag may be inferred to be equal to 0.

Further referring to FIG. 2, sps_6param_affine_enabled_flag_equal to 1 may specify that the 6-parameter affine model based motion compensation is enabled for the CLVS.

Still referring to FIG. 2, translational CPMVs may be signaled in PPS. Control points may be predefined. For example, control point MV 0 may be relative to a top left corner of a picture, MV 1 may be relative to a top right corner, and MV 3 may be relative to a bottom left corner of the picture. Table 4 illustrates an example approach for signaling CPMV data depending on a motion model used.

In an exemplary embodiment, and still referring to FIG. 2, an array amvr_precision_idx, which may be signaled in coding unit, coding tree, or the like, may specify a resolution AmvrShift of a motion vector difference, which may be defined as a non-limiting example as shown in Table 7 as shown below. Array indices x0, y0 may specify the location (x0, y0) of a top-left luma sample of a considered coding block relative to a top-left luma sample of the picture; when amvr_precision_idx[x0][y0] is not present, it may be inferred to be equal to 0. Where an inter_affine_flag[x0][y0] is equal to 0, variables MvdL0[x0][y0][0], MvdL0[x0][y0][1], MvdL1[x0][y0][0], MvdL1[x0][y0][1] representing modsion vector difference values corresponding to consered block, may be modified by shifting such values by AmvrShift, for instance using MvdL0[x0][y0][0]=MvdL0[x0][y0][0]<<AmvrShift; MvdL0[x0][y0][1]=MvdL0[x0][y0][1]<<AmvrShift; MvdL1[x0][y0][0]=MvdL1[x0][y0][0]<<AmvrShift; and MvdL1[x0][y0][1]=MvdL1[x0][y0][1]<<AmvrShift. Where inter_affine_flag[x0][y0] is equal to 1, variables MvdCpL0[x0][y0][0][0], MvdCpL0[x0][y0][0][1], MvdCpL0[x0][y0][1][0], MvdCpL0[x0][y0][1][1], MvdCpL0[x0][y0][2][0] and MvdCpL0[x0][y0][2][1] may be modified via shifting, for instance as follows: MvdCpL0[x0][y0][0][0]=MvdCpL0[x0][y0][0][0]<<AmvrShift; MvdCpL1[x0][y0] [0][1]=MvdCpL1[x0][y0][0][1]<<AmvrShift; MvdCpL0[x0][y0][1][0]=MvdCpL0[x0][y0][1][0]<<AmvrShift; MvdCpL1[x0][y0] [1][1]=MvdCpL1[x0][y0][1][1]<<AmvrShift; MvdCpL0[x0][y0][2][0]=MvdCpL0[x0][y0][2][0]<<AmvrShift; and MvdCpL1[x0][y0] [2][1]=MvdCpL1[x0][y0][2][1]<<AmvrShift

AmvrShift inter_af- amvr_preci- inter_af- CuPredMode[ chType ][ x0 ][ y0 ] = = fine_flag = =0&&CuPredMode[ chType ][ x0 ][ y0 ] != amvr_flag sion_i fine_flag = =1 MODE_IBC ) MODE_IBC 0 — 2 (¼ luma sample) — 2 (¼ luma sample) 1 0 0 ( 1/16 luma sample) 4 (1 luma sample) 3 (½ luma sample) 1 1 4 (1 luma sample) 6 (4 luma samples) 4 (1 luma sample) 1 2 — — 6 (4 luma samples)

Further referring to FIG. 2, global motion may be relative to a previously coded frame. When only one set of global motion parameters are present, motion may be relative to a frame that is presented immediately before a current frame.

Still referring to FIG. 2, global motion may represent a dominant motion in a frame. Many blocks in a frame may be likely to have a motion that is same as very similar to a global motion. Exceptions may be blocks with local motion. Keeping block motion compensation compatible with global motion may reduce encoder complexity and decoder complexity and may improve compression efficiency.

In some implementations, and continuing to refer to FIG. 2, if global motion is signaled in a header such as a PPS or an SPS, a motion model in the SPS may be applied to all blocks in a picture. For example, if global motion uses translational motion (e.g., motion model=0), all prediction units (PUs) in a frame may also be limited to translational motion (e.g., motion model=0). In this case, adaptive motion models may not be used. This may also be signaled in SPS using a use_gm_constrained_motion_models flag. When this flag is set to 1, adaptive motion models may not be used in a decoder; instead, a single motion model may be used for all PUs.

Still referring to FIG. 2, in some implementations of the current subject matter, motion signaling may not change over PUs. Instead, a fixed motion model may be used by signaling a motion model once in SPS. Such an approach may replace global motion. Use of fixed motion model may be specified at an encoder to reduce complexity; for instance, the encoder may be limited to translational model, which can be advantageous for low power devices, such as low computational power devices. For example, affine motion models may not be used, for instance as specified in an encoder profile. Such an example may be useful for real-time applications, such as video conferencing, information of things (IoT) infrastructure, security cameras, and the like. By using a fixed motion model, there may be no need to include excess signaling in the bitstream.

Further referring to FIG. 2, the current subject matter is not limited to coding techniques utilizing global motion but may apply to a broad range of coding techniques.

As described above, and still referring to FIG. 2, global motion may represent a dominant motion in a frame. Many blocks in a frame may be likely to have a motion that is same as very similar to global motion, with exception of blocks with local motion. Keeping block motion compensation compatible with global motion may reduce encoder complexity and decoder complexity, and may improve compression efficiency.

Still referring to FIG. 2, rather than constraining the motion of each block to be the same as a motion model, such as without limitation a global motion model, signaled in a header, such as a PPS or an SPS, motion model applied to each block in a frame may be constrained to similar motion models. Similar motion models may include those models that have the same complexity or are less complex. For example, the following three models are shown in the first column of table 5 in increasing order of complexity.

Global Motion Model Motion Model Use in Inter Coding Translational (MM = 0) Translational 4-parameter affine (MM = 1) Translational or 4-parameter affine 6-parameter affine (MM = 2) Translational or 4-parameter affine or 6-parameter affine

Still referring to FIG. 2, the second column of table 5 shows permissible motion models for a block to use for inter coding. For example, some implementations of the current subject matter may allow a PU to take on a motion model where an index of the motion model is less than or equal to an index of the motion model for global motion.

With continued reference to FIG. 2, keeping motion models compatible with global motion may allow for use of global motion control points as motion vector prediction candidates. Global motion CPMVs may represent motion similar to motion of PUs and make good MV prediction candidates.

FIG. 3 is a process flow diagram illustrating an exemplary embodiment of a process 300 of the applying a similar motion model to a global motion model signaled in a header.

At step 305, a bitstream is received by a decoder. A current block may be contained within a bitstream that a decoder receives. Bitstream may include, for example, data found in a stream of bits that is an input to a decoder when using data compression. Bitstream may include information necessary to decode a video. Receiving may include extracting and/or parsing a block and associated signaling information from bit stream. In some implementations, a current block may include a coding tree unit (CTU), a coding unit (CU), and/or a prediction unit (PU).

At step 310, and still referring to FIG. 3, a header associated with a current frame and including a signal characterizing that global motion is enabled and further characterizing parameters of a motion model can be extracted. At step 315, a current frame may be decoded. Decoding may include using a motion model for each current block having a complexity that is less than or equal to a complexity of a global motion model.

FIG. 4 is a system block diagram illustrating an exemplary embodiment of a decoder 2400 capable of decoding a bitstream including applying a similar motion model to a global motion model signaled in a header. Decoder 400 may include an entropy decoder processor 404, an inverse quantization and inverse transformation processor 408, a deblocking filter 412, a frame buffer 416, a motion compensation processor 420 and/or an intra prediction processor 424.

In operation, and still referring to FIG. 4, bit stream 428 may be received by decoder 400 and input to entropy decoder processor 404, which may entropy decode portions of bit stream into quantized coefficients. Quantized coefficients may be provided to inverse quantization and inverse transformation processor 408, which may perform inverse quantization and inverse transformation to create a residual signal, which may be added to an output of motion compensation processor 420 or intra prediction processor 424 according to a processing mode. An output of the motion compensation processor 420 and intra prediction processor 424 may include a block prediction based on a previously decoded block. A sum of prediction and residual may be processed by deblocking filter 412 and stored in a frame buffer 416.

FIG. 3 is a process flow diagram illustrating an exemplary embodiment of a process 300 of encoding a video including applying a similar motion model to a global motion model signaled in a header according to some aspects of the current subject matter that may reduce encoding complexity while increasing compression efficiency. At step 305, a video frame may undergo initial block segmentation, for example, using a tree-structured macro block partitioning scheme that may include partitioning a picture frame into CTUs and CUs.

At step 310, and still referring to FIG. 3, a global motion for a current block or frame may be determined. At step 315, block may be encoded and included in bitstream. Encoding may include signaling in a header that for all blocks in a frame, a similar motion model to a global motion model should be applied. Encoding may include utilizing inter prediction and intra prediction modes, for example.

FIG. 6 is a system block diagram illustrating an exemplary embodiment of a video encoder 600 capable of applying a similar motion model to a global motion model signaled in a header. Example video encoder 600 may receive an input video 604, which may be initially segmented and/or dividing according to a processing scheme, such as a tree-structured macro block partitioning scheme (e.g., quad-tree plus binary tree). An example of a tree-structured macro block partitioning scheme may include partitioning a picture frame into large block elements called coding tree units (CTU). In some implementations, each CTU may be further partitioned one or more times into a number of sub-blocks called coding units (CU). A final result of this portioning may include a group of sub-blocks that may be called predictive units (PU). Transform units (TU) may also be utilized.

Still referring to FIG. 6, example video encoder 600 may include an intra prediction processor 612, a motion estimation/compensation processor 612 (also referred to as an inter prediction processor) capable of constructing a motion vector candidate list including adding a single global motion vector candidate to the motion vector candidate list, a transform/quantization processor 616, an inverse quantization/inverse transform processor 620, an in-loop filter 624, a decoded picture buffer 628, and/or an entropy coding processor 632. Bit stream parameters may be input to entropy coding processor 632 for inclusion in an output bit stream 636.

In operation, and with continued reference to FIG. 6, for each block of a frame of input video 604, whether to process block via intra picture prediction or using motion estimation/compensation may be determined. Block may be provided to intra prediction processor 608 or motion estimation/compensation processor 612. If block is to be processed via intra prediction, intra prediction processor 608 may perform processing to output a predictor. If block is to be processed via motion estimation/compensation, motion estimation/compensation processor 612 may perform processing including constructing a motion vector candidate list including adding a single global motion vector candidate to the motion vector candidate list, if applicable.

Still referring to FIG. 6, a residual may be formed by subtracting predictor from input video. Residual may be received by transform/quantization processor 616, which may perform transformation processing (e.g., discrete cosine transform (DCT)) to produce coefficients, which may be quantized. Quantized coefficients and any associated signaling information may be provided to entropy coding processor 632 for entropy encoding and inclusion in an output bit stream 636. Entropy encoding processor 632 may support encoding of signaling information related to encoding a current block. In addition, quantized coefficients may be provided to inverse quantization/inverse transformation processor 620, which may reproduce pixels, which may be combined with predictor and processed by in loop filter 624, an output of which may be stored in decoded picture buffer 628 for use by motion estimation/compensation processor 612 that is capable of constructing a motion vector candidate list including adding a single global motion vector candidate to the motion vector candidate list.

Further referencing FIG. 6, although a few variations have been described in detail above, other modifications or additions are possible. For example, in some implementations, current blocks may include any symmetric blocks (8×8, 16×16, 32×32, 64×64, 128×128, and the like) as well as any asymmetric block (8×4, 16×8, and the like).

In some implementations, and still referring to FIG. 6, a quadtree plus binary decision tree (QTBT) may be implemented. In QTBT, at a Coding Tree Unit level, partition parameters of QTBT may be dynamically derived to adapt to local characteristics without transmitting any overhead. Subsequently, at a Coding Unit level, a joint-classifier decision tree structure may eliminate unnecessary iterations and control risk of false prediction. In some implementations, LTR frame block update mode may be available as an additional option available at every leaf node of a QTBT.

In some implementations, and still referring to FIG. 6, additional syntax elements may be signaled at different hierarchy levels of a bitstream. For example, a flag may be enabled for an entire sequence by including an enable flag coded in a Sequence Parameter Set (SPS). Further, a CTU flag may be coded at a coding tree unit (CTU) level.

It is to be noted that any one or more of the aspects and embodiments described herein may be conveniently implemented using digital electronic circuitry, integrated circuitry, specially designed application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs) computer hardware, firmware, software, and/or combinations thereof, as realized and/or implemented in one or more machines (e.g., one or more computing devices that are utilized as a user computing device for an electronic document, one or more server devices, such as a document server, etc.) programmed according to the teachings of the present specification, as will be apparent to those of ordinary skill in the computer art. These various aspects or features may include implementation in one or more computer programs and/or software that are executable and/or interpretable on a programmable system including at least one programmable processor, which may be special or general purpose, coupled to receive data and instructions from, and to transmit data and instructions to, a storage system, at least one input device, and at least one output device. Appropriate software coding may readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent to those of ordinary skill in the software art. Aspects and implementations discussed above employing software and/or software modules may also include appropriate hardware for assisting in the implementation of the machine executable instructions of the software and/or software module.

Such software may be a computer program product that employs a machine-readable storage medium. A machine-readable storage medium may be any medium that is capable of storing and/or encoding a sequence of instructions for execution by a machine (e.g., a computing device) and that causes the machine to perform any one of the methodologies and/or embodiments described herein. Examples of a machine-readable storage medium include, but are not limited to, a magnetic disk, an optical disc (e.g., CD, CD-R, DVD, DVD-R, etc.), a magneto-optical disk, a read-only memory “ROM” device, a random access memory “RAM” device, a magnetic card, an optical card, a solid-state memory device, an EPROM, an EEPROM, Programmable Logic Devices (PLDs), and/or any combinations thereof. A machine-readable medium, as used herein, is intended to include a single medium as well as a collection of physically separate media, such as, for example, a collection of compact discs or one or more hard disk drives in combination with a computer memory. As used herein, a machine-readable storage medium does not include transitory forms of signal transmission.

Such software may also include information (e.g., data) carried as a data signal on a data carrier, such as a carrier wave. For example, machine-executable information may be included as a data-carrying signal embodied in a data carrier in which the signal encodes a sequence of instruction, or portion thereof, for execution by a machine (e.g., a computing device) and any related information (e.g., data structures and data) that causes the machine to perform any one of the methodologies and/or embodiments described herein.

Examples of a computing device include, but are not limited to, an electronic book reading device, a computer workstation, a terminal computer, a server computer, a handheld device (e.g., a tablet computer, a smartphone, etc.), a web appliance, a network router, a network switch, a network bridge, any machine capable of executing a sequence of instructions that specify an action to be taken by that machine, and any combinations thereof. In one example, a computing device may include and/or be included in a kiosk.

FIG. 7 shows a diagrammatic representation of one embodiment of a computing device in the exemplary form of a computer system 700 within which a set of instructions for causing a control system to perform any one or more of the aspects and/or methodologies of the present disclosure may be executed. It is also contemplated that multiple computing devices may be utilized to implement a specially configured set of instructions for causing one or more of the devices to perform any one or more of the aspects and/or methodologies of the present disclosure. Computer system 700 includes a processor 704 and a memory 708 that communicate with each other, and with other components, via a bus 712. Bus 712 may include any of several types of bus structures including, but not limited to, a memory bus, a memory controller, a peripheral bus, a local bus, and any combinations thereof, using any of a variety of bus architectures.

Memory 708 may include various components (e.g., machine-readable media) including, but not limited to, a random-access memory component, a read only component, and any combinations thereof. In one example, a basic input/output system 716 (BIOS), including basic routines that help to transfer information between elements within computer system 700, such as during start-up, may be stored in memory 708. Memory 708 may also include (e.g., stored on one or more machine-readable media) instructions (e.g., software) 720 embodying any one or more of the aspects and/or methodologies of the present disclosure. In another example, memory 708 may further include any number of program modules including, but not limited to, an operating system, one or more application programs, other program modules, program data, and any combinations thereof.

Computer system 700 may also include a storage device 724. Examples of a storage device (e.g., storage device 724) include, but are not limited to, a hard disk drive, a magnetic disk drive, an optical disc drive in combination with an optical medium, a solid-state memory device, and any combinations thereof. Storage device 724 may be connected to bus 712 by an appropriate interface (not shown). Example interfaces include, but are not limited to, SCSI, advanced technology attachment (ATA), serial ATA, universal serial bus (USB), IEEE 1394 (FIREWIRE), and any combinations thereof. In one example, storage device 724 (or one or more components thereof) may be removably interfaced with computer system 700 (e.g., via an external port connector (not shown)). Particularly, storage device 724 and an associated machine-readable medium 728 may provide nonvolatile and/or volatile storage of machine-readable instructions, data structures, program modules, and/or other data for computer system 700. In one example, software 720 may reside, completely or partially, within machine-readable medium 728. In another example, software 720 may reside, completely or partially, within processor 704.

Computer system 700 may also include an input device 732. In one example, a user of computer system 700 may enter commands and/or other information into computer system 700 via input device 732. Examples of an input device 732 include, but are not limited to, an alpha-numeric input device (e.g., a keyboard), a pointing device, a joystick, a gamepad, an audio input device (e.g., a microphone, a voice response system, etc.), a cursor control device (e.g., a mouse), a touchpad, an optical scanner, a video capture device (e.g., a still camera, a video camera), a touchscreen, and any combinations thereof. Input device 732 may be interfaced to bus 712 via any of a variety of interfaces (not shown) including, but not limited to, a serial interface, a parallel interface, a game port, a USB interface, a FIREWIRE interface, a direct interface to bus 712, and any combinations thereof. Input device 732 may include a touch screen interface that may be a part of or separate from display 736, discussed further below. Input device 732 may be utilized as a user selection device for selecting one or more graphical representations in a graphical interface as described above.

A user may also input commands and/or other information to computer system 700 via storage device 724 (e.g., a removable disk drive, a flash drive, etc.) and/or network interface device 740. A network interface device, such as network interface device 740, may be utilized for connecting computer system 700 to one or more of a variety of networks, such as network 744, and one or more remote devices 748 connected thereto. Examples of a network interface device include, but are not limited to, a network interface card (e.g., a mobile network interface card, a LAN card), a modem, and any combination thereof. Examples of a network include, but are not limited to, a wide area network (e.g., the Internet, an enterprise network), a local area network (e.g., a network associated with an office, a building, a campus or other relatively small geographic space), a telephone network, a data network associated with a telephone/voice provider (e.g., a mobile communications provider data and/or voice network), a direct connection between two computing devices, and any combinations thereof. A network, such as network 744, may employ a wired and/or a wireless mode of communication. In general, any network topology may be used. Information (e.g., data, software 720, etc.) may be communicated to and/or from computer system 700 via network interface device 740.

Computer system 700 may further include a video display adapter 752 for communicating a displayable image to a display device, such as display device 736. Examples of a display device include, but are not limited to, a liquid crystal display (LCD), a cathode ray tube (CRT), a plasma display, a light emitting diode (LED) display, and any combinations thereof. Display adapter 752 and display device 736 may be utilized in combination with processor 704 to provide graphical representations of aspects of the present disclosure. In addition to a display device, computer system 700 may include one or more other peripheral output devices including, but not limited to, an audio speaker, a printer, and any combinations thereof. Such peripheral output devices may be connected to bus 712 via a peripheral interface 756. Examples of a peripheral interface include, but are not limited to, a serial port, a USB connection, a FIREWIRE connection, a parallel connection, and any combinations thereof.

The foregoing has been a detailed description of illustrative embodiments of the invention. Various modifications and additions can be made without departing from the spirit and scope of this invention. Features of each of the various embodiments described above may be combined with features of other described embodiments as appropriate in order to provide a multiplicity of feature combinations in associated new embodiments. Furthermore, while the foregoing describes a number of separate embodiments, what has been described herein is merely illustrative of the application of the principles of the present invention. Additionally, although particular methods herein may be illustrated and/or described as being performed in a specific order, the ordering is highly variable within ordinary skill to achieve embodiments as disclosed herein. Accordingly, this description is meant to be taken only by way of example, and not to otherwise limit the scope of this invention.

In the descriptions above and in the claims, phrases such as “at least one of” or “one or more of” may occur followed by a conjunctive list of elements or features. The term “and/or” may also occur in a list of two or more elements or features. Unless otherwise implicitly or explicitly contradicted by the context in which it is used, such a phrase is intended to mean any of the listed elements or features individually or any of the recited elements or features in combination with any of the other recited elements or features. For example, the phrases “at least one of A and B;” “one or more of A and B;” and “A and/or B” are each intended to mean “A alone, B alone, or A and B together.” A similar interpretation is also intended for lists including three or more items. For example, the phrases “at least one of A, B, and C;” “one or more of A, B, and C;” and “A, B, and/or C” are each intended to mean “A alone, B alone, C alone, A and B together, A and C together, B and C together, or A and B and C together.” In addition, use of the term “based on,” above and in the claims is intended to mean, “based at least in part on,” such that an unrecited feature or element is also permissible.

The subject matter described herein can be embodied in systems, apparatus, methods, and/or articles depending on the desired configuration. The implementations set forth in the foregoing description do not represent all implementations consistent with the subject matter described herein. Instead, they are merely some examples consistent with aspects related to the described subject matter. Although a few variations have been described in detail above, other modifications or additions are possible. In particular, further features and/or variations can be provided in addition to those set forth herein. For example, the implementations described above can be directed to various combinations and sub-combinations of the disclosed features and/or combinations and sub-combinations of several further features disclosed above. In addition, the logic flows depicted in the accompanying figures and/or described herein do not necessarily require the particular order shown, or sequential order, to achieve desirable results. Other implementations may be within the scope of the following claims. 

What is claimed is:
 1. A decoder, the decoder comprising circuitry configured to: receive a bitstream; extract from the bitstream a parameter set associated with a coded picture in the bitstream, wherein the parameter set includes a signal indicating that a global motion model is enabled, the global motion model comprising an affine motion model using control point motion vectors; and decode the coded picture using a motion model for each block of the coded picture, the motion model for each block having a complexity that is no greater than a complexity of the global motion model, wherein the complexity of a motion model is a function of a number of motion vectors needed to decode a block.
 2. The decoder of claim 1, wherein the parameter set includes a sequence parameter set (SPS).
 3. The decoder of claim 1, wherein the global motion model includes a four-parameter affine motion model.
 4. The decoder of claim 1, wherein the global motion model includes a six-parameter affine motion model.
 5. The decoder of claim 1, wherein each block is a coding tree unit.
 6. The decoder of claim 1, wherein each block is a coding unit.
 7. The decoder of claim 1, wherein each block in the coded picture is decoded using a translational motion model.
 8. The decoder of claim 1, wherein each block of a first region of the coded picture is decoded using the global motion model and each block in a second region of the coded picture is decoded using a translational motion model.
 9. The decoder of claim 1, wherein the global motion model is a six-parameter affine motion model, each block in a first region of the coded picture is decoded using the global motion model, and each blocks of a second region of the coded picture is decoded using one or both of a four-parameter affine motion model and a translational motion model.
 10. A method comprising: receiving, by a decoder, a bitstream; extracting, by the decoder, from the bitstream a parameter set associated with a coded picture in the bitstream, wherein the parameter set includes a signal indicating that a global motion model is enabled, the global motion model comprising an affine motion model using control point motion vectors; and decoding, by the decoder, the coded picture using a motion model for each block having a complexity that is no greater than a complexity of the global motion model, wherein the complexity of a motion model is a function of a number of motion vectors needed to decode a block.
 11. The method of claim 10, wherein the parameter set includes a sequence parameter set (SPS).
 12. The method of claim 10, wherein the global motion model includes a four-parameter affine motion model.
 13. The method of claim 10, wherein the global motion model includes a six-parameter affine motion model.
 14. The method of claim 10, wherein each block is a coding tree unit.
 15. The method of claim 10, wherein each block is a coding unit.
 16. The method of claim 10, wherein each block in the coded picture is decoded using a translational motion model.
 17. The method of claim 10, wherein each block of a first region of the coded picture is decoded using the global motion model and each block in a second region of the coded picture is decoded using a translational motion model.
 18. The method of claim 10, wherein the global motion model is a six-parameter affine motion model, each block in a first region of the coded picture is decoded using the global motion model, and each block of a second region of the coded picture is decoded using one or both of a four-parameter affine motion model and a translational motion model. 